Typically, there are spectacle lenses having an alignment reference mark formed to identify a distance portion design reference point (hereinafter, simply referred to as “design reference point”) defined in a JIS standard (JIS T 7330). An example of this type of spectacle lens includes a progressive power spectacle lens. In a case of the progressive power spectacle lens, power distribution is more complicated than a single focus lens and the like. Therefore, it is difficult to precisely identify the design reference point with a lens meter or the like after finishing processing of a lens surface is completed. Further, the design reference point is close to a position where a gaze passes through when a wearer of the spectacle views a distant point and thus if the alignment reference mark is formed on the design reference point, the alignment reference mark becomes an obstacle of the distant view. Further, a horizontal axis (an axis in a direction of 0 to 180 degrees) and a vertical axis (an axis in a direction of 90 to 270 degrees) are set to the progressive power spectacle lens centering around the design reference point. Therefore, the design reference point cannot be identified with only one alignment reference mark. Therefore, two alignment reference marks are formed on the progressive power spectacle lens with equal spaces from the design reference point to the right and left (in the horizontal axis direction). Providing the two alignment reference marks on the progressive power lens is defined in a JIS standard (JIS T 7315).
Conventionally, a lens called semi-finished lens is typically used, in which an object side surface (convex surface side) of a progressive power spectacle lens is a progressive surface and the convex surface side is optically finished. Therefore, a polishing jig is mounted on the convex surface of the semi-finished lens, and a concave surface is finished to have a desired surface shape.
Meanwhile, a spectacle lens that has undergone the above finishing processing and have both surfaces become final optical surfaces (hereinafter, the spectacle lens is also referred to as “uncut lens”) undergoes lens shape processing to be finally fit into a spectacle frame. To perform the lens shape processing, a lens shape processing lens holder is mounted to the spectacle lens, using the alignment reference marks on the spectacle lens as references, in a block process that is a preprocess of the lens shape processing. To be specific, a center position (hereinafter, referred to as “holder mounting center position”) where the lens holder should be mounted on the convex surface of the spectacle lens is determined, and the lens holder is mounted to the holder mounting center position. At that time, the holder mounting center position is determined by visually recognizing (imaging) the alignment reference marks from the convex surface side of the spectacle lens. Further, in the lens shape processing process thereafter, the spectacle lens to which the lens holder has been mounted is set to a lens shape processor, and then the lens shape processing (including edge grinding processing, lens edging processing, and the like) is performed using a processing tool included in the lens shape processor, so that a lens that has undergone the lens shape processing is completed.
Conventionally, as a technology of determining the holder mounting center position using the alignment reference marks, a technology described in Patent Literature 1 is known, for example. This conventional technology determines the holder mounting center position by imaging the alignment reference marks formed on one lens surface of the spectacle lens from a side of the lens surface where the alignment reference marks are formed, using two imaging units.